hwui: change TRIM_COMPLETE to TRIM_BACKGROUND

In preparation for lifecycle-consistent TRIM_MEMORY callbacks, be more
aggressive with TRIM_BACKGROUND.

Test: android.platform.test.scenario.launcher.FlingAllAppsMicrobenchmark
Bug: 253914117
Change-Id: I4c8b3331312f198dbf3f74495b8412873d1ca302
1 file changed