Cache the build serial instead of querying every time.

Also only return the original serial number if the
targetSandboxVersion is less than 2, not just != 2.

Change-Id: Ie17666798922bdac10a79bbc00934b311b816818
Fixes: 63808327
Test: run cts-dev -m CtsAppSecurityHostTestCases -t
android.appsecurity.cts.EphemeralTest#testBuildSerialUnknown