Add flag for optimized resource shrinking with R8

If the flag is set we will:
 - pass --optimized-resource-shrinking to the r8 invocation
 - use non final fields for the generated R classes
 - not pass the aapt2 generated proguard rules, R8 will do the tracing of xml files

Bug: 325905703
Test: This is simply passing the flag through to R8, this is off by default
Change-Id: Ib2043f3201578c3bcd39c1de9a524fd78f6d795c