Allow passing --legacy in aaptflags

--legacy will soon no longer be passed by default to all aapt2
compiles.  Allow it to be specified in aaptflags by passing it to
aapt2 compile when it is present and filtering it out from the
flags passed to aapt2 link.

Bug: 135597368
Test: m java
Change-Id: Ib65d1a9b7c32ae4ff5ab6f58e66aedfb5f296712