Reduce duplicate strings due to the package cache.

Change from the previous attempt:
- Fixed the helper class.  The original version had a few bugs.
- Bundle.readFromParcel() now handles a Parcel with a read-write helper
properly.

** Comparison **
The following charts are the actual measurement with and without the fix,
using "dumpsys system".
- The red bar is "total private dirty".
- The X axsis is time since boot.

Without fix:
- #1 First boot:
-- https://docs.google.com/spreadsheets/d/1CbmU8cQQQw7n7tyqbZi3beRHNuzqcmJgdvzDpi40Q1I/edit#gid=1971317391
-- Private dirty stabilizes at ~16.8M.
-- Loading system packages took 1.8 seconds.

- #2 Second boot:
-- https://docs.google.com/spreadsheets/d/1CbmU8cQQQw7n7tyqbZi3beRHNuzqcmJgdvzDpi40Q1I/edit#gid=982210726
-- Private dirty stabilizes at ~17.5M.
-- Loading system packages took 0.5 seconds.

With fix:
- #3 First boot:
-- https://docs.google.com/spreadsheets/d/1R6lL0AnAp93HnrqWujJFNgOjj6wvGicgDlbDAevbc3g/edit#gid=791764875
-- Private dirty stabilizes at around the same level as #1.
-- Loading system packages took 1.9 seconds.

- #4 Second boot:
-- https://docs.google.com/spreadsheets/d/1CbmU8cQQQw7n7tyqbZi3beRHNuzqcmJgdvzDpi40Q1I/edit#gid=1820894299
-- Private dirty stabilizes at around the same level as #1.
-- Loading system packages took 0.7 seconds.

Package manager start up time with and without the fix:
- (Ignored ones that are too fast; probably the thermal throttling didn't kick in.)
- https://docs.google.com/spreadsheets/d/1CbmU8cQQQw7n7tyqbZi3beRHNuzqcmJgdvzDpi40Q1I/edit#gid=499396796
- Before: 3.5 seconds (average of 5 reboots)
- After: 3.6 seconds (average of 5 reboots)

Package scan speed comparison:
- With the fix, first boot.
08-03 08:49:56.851  1000   779   779 I PackageManager: Finished scanning system apps. Time: 2133 ms, packageCount: 143 , timePerPackage: 14 , cached: 0
08-03 08:49:56.971  1000   779   779 I PackageManager: Finished scanning non-system apps. Time: 121 ms, packageCount: 11 , timePerPackage: 11 , cached: 0

- With the fix, second boot.
08-03 08:53:29.387  1000   779   779 I PackageManager: Finished scanning system apps. Time: 484 ms, packageCount: 143 , timePerPackage: 3 , cached: 143
08-03 08:53:29.424  1000   779   779 I PackageManager: Finished scanning non-system apps. Time: 37 ms, packageCount: 11 , timePerPackage: 3 , cached: 11

** Conclusion **
- This CL wil slightly slow down the boot time (0.2 seconds on a thermal-throttled bullhead), but
the system server's ram consumption will go down to the no-cache level.

- Using the package cache is still faster than not using it.

Test: build, boot, reboot, adb-install, reboot
Test: bit FrameworksCoreTests:android.content.pm.PackageParserTest
Test: bit FrameworksServicesTests:com.android.server.pm.PackageParserTest
Test: cts-tradefed run cts-dev --skip-device-info --skip-preconditions --skip-system-status-check com.android.compatibility.common.tradefed.targetprep.NetworkConnectivityChecker -a armeabi-v7a -l INFO -m CtsOsTestCases -t android.os.cts.BundleTest

Bug: 64112468
Change-Id: I30691a032cb1dd1c7f6c1966a096c2f0d07a09cb