Add build flag for shrinking resources by default

This enables resource shrinking as the default optimization setting for
app targets, when unspecified. This will likely involve explicitly
opting out a subset of more problematic apps so we can safely make the
default transition.

Bug: 432099500
Test: presubmit
Flag: build.RELEASE_R8_SHRINK_RESOURCES_BY_DEFAULT
Change-Id: I2c38980bd602ed3271604bd2fe3cf329a6089cba