Add CanWriteToDalvikCache() check before writing a boot marker or image cache.

A new zygote process, the webview_zygote, will spawn isolated_app children,
but the process itself does not run as root and cannot write to the dalvik
cache. In order to not print SELinux avc errors, check for file-write
permission when testing to see if a boot marker or cache image or should be
written.

Test: m test-art
Test: angler boots

Bug: 21643067
Change-Id: I9f797fe7332a21ef4c6b83d2210b1673af09de1b