Trim font caches when keyguard goes away

Right now we send a trim UI_HIDDEN command to trim the resources used by
lockscreen after unlock, but that does not clean up any font caches
generated by e.g. lockscreen clock animation. This can result in
significant memory bloat for user apps once the device is unlock.

This trims font caches after unlock as well to increase availability of
resources for apps.

Bug:275486055
Bug:278225145
Bug:283203305

Test: newly added unit tests for ResourceTrimmer
      ran LockscreenWithSwipeMicrobenchmark and confirmed lower memory
      use
Change-Id: I84637049a106a94bf9f06c5261c2941b55db4ba0